[Bug 1767223] [NEW] multilib selects wrong start files

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The multilib setup is not working in the bionic version of gcc-arm-none-
eabi (gcc v6.3.1). For example, if I build a project with
"-mcpu=cortex-m4" or "-march=armv7e-m" and link with:

arm-none-eabi-gcc -o someoutput.elf  object1.o object2.o -mthumb -Wl
,--gc-sections -ffast-math -march=armv7-m -Tlinker_script.ld

I get as output:

/usr/lib/gcc/arm-none-eabi/6.3.1/../../../arm-none-eabi/bin/ld: error: /usr/lib/gcc/arm-none-eabi/6.3.1/../../../arm-none-eabi/lib/crt0.o: Conflicting CPU architectures 13/1
/usr/lib/gcc/arm-none-eabi/6.3.1/../../../arm-none-eabi/bin/ld: failed to merge target specific data of file /usr/lib/gcc/arm-none-eabi/6.3.1/../../../arm-none-eabi/lib/crt0.o

arm-none-eabi-readelf -A on any of my object files produces:

Attribute Section: aeabi
File Attributes
  Tag_CPU_name: "Cortex-M4"
  Tag_CPU_arch: v7E-M
  Tag_CPU_arch_profile: Microcontroller
  Tag_THUMB_ISA_use: Thumb-2
  Tag_ABI_PCS_wchar_t: 4
  Tag_ABI_FP_number_model: Finite
  Tag_ABI_align_needed: 8-byte
  Tag_ABI_align_preserved: 8-byte, except leaf SP
  Tag_ABI_enum_size: small
  Tag_ABI_optimization_goals: Prefer Speed
  Tag_CPU_unaligned_access: v6

But "arm-none-eabi-readelf -A /usr/lib/gcc/arm-none-eabi/6.3.1
/../../../arm-none-eabi/lib/crt0.o" gives

Attribute Section: aeabi
File Attributes
  Tag_CPU_arch: v4
  Tag_ARM_ISA_use: Yes

If I define "void _init(void){}" and use -nostartfiles, compilation
finishes but the resulting binary does not work. I have not examined the
output file but it appears that it has somehow linked to code containing
ARM as well as thumb code:

# readelf -A someoutput.elf
Attribute Section: aeabi
File Attributes
  Tag_CPU_name: "Cortex-M4"
  Tag_CPU_arch: v7E-M
  Tag_CPU_arch_profile: Microcontroller
  Tag_ARM_ISA_use: Yes
  Tag_THUMB_ISA_use: Thumb-2
  Tag_ABI_PCS_wchar_t: 4
  Tag_ABI_FP_denormal: Needed
  Tag_ABI_FP_exceptions: Needed
  Tag_ABI_FP_number_model: IEEE 754
  Tag_ABI_align_needed: 8-byte
  Tag_ABI_align_preserved: 8-byte, except leaf SP
  Tag_ABI_enum_size: small
  Tag_ABI_optimization_goals: Prefer Speed
  Tag_CPU_unaligned_access: v6

I can work around the problem by manually specifying compatible versions
of libc and libgcc with -nostdlib:

-nostdlib /usr/lib/arm-none-eabi/newlib/thumb/libc.a /usr/lib/gcc/arm-
none-eabi/6.3.1/thumb/v7e-m/libgcc.a

This produces a working binary. However, there is still something wrong
because readelf -A still includes "Tag_ARM_ISA_use: Yes":

Attribute Section: aeabi
File Attributes
  Tag_CPU_name: "Cortex-M4"
  Tag_CPU_arch: v7E-M
  Tag_CPU_arch_profile: Microcontroller
  Tag_ARM_ISA_use: Yes
  Tag_THUMB_ISA_use: Thumb-2
  Tag_ABI_PCS_wchar_t: 4
  Tag_ABI_FP_denormal: Needed
  Tag_ABI_FP_exceptions: Needed
  Tag_ABI_FP_number_model: IEEE 754
  Tag_ABI_align_needed: 8-byte
  Tag_ABI_align_preserved: 8-byte, except leaf SP
  Tag_ABI_enum_size: small
  Tag_ABI_optimization_goals: Prefer Speed
  Tag_CPU_unaligned_access: v6

Previously, using xenial and gcc v4, readelf gives the following output on the output binary:
Attribute Section: aeabi
File Attributes
  Tag_CPU_name: "7E-M"
  Tag_CPU_arch: v7E-M
  Tag_CPU_arch_profile: Microcontroller
  Tag_THUMB_ISA_use: Thumb-2
  Tag_ABI_PCS_wchar_t: 4
  Tag_ABI_FP_denormal: Needed
  Tag_ABI_FP_exceptions: Needed
  Tag_ABI_FP_number_model: IEEE 754
  Tag_ABI_align_needed: 8-byte
  Tag_ABI_enum_size: small
  Tag_ABI_optimization_goals: Prefer Speed
  Tag_CPU_unaligned_access: v6

(Also, what happened to gdb-arm-none-eabi?)

** Affects: newlib (Ubuntu)
     Importance: Undecided
         Status: Confirmed

-- 
multilib selects wrong start files
https://bugs.launchpad.net/bugs/1767223
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