Add unwind information to external assembly

[Tiwalun]

Add unwind information to the external assembly files, as discussed in #215.

The .debug_frame section is now present:

cortex-m.o   (ex bin/thumbv6m-none-eabi.a):
section              size   addr
.text                 0x0    0x0
.data                 0x0    0x0
.bss                  0x0    0x0
.text.__bkpt          0x4    0x0
.text.__control_r     0x6    0x0
.text.__control_w     0x6    0x0
.text.__cpsid         0x4    0x0
.text.__cpsie         0x4    0x0
.text.__delay         0x8    0x0
.text.__dmb           0x6    0x0
.text.__dsb           0x6    0x0
.text.__isb           0x6    0x0
.text.__msp_r         0x6    0x0
.text.__msp_w         0x6    0x0
.text.__nop           0x2    0x0
.text.__primask       0x6    0x0
.text.__psp_r         0x6    0x0
.text.__psp_w         0x6    0x0
.text.__sev           0x4    0x0
.text.__udf           0x2    0x0
.text.__wfe           0x4    0x0
.text.__wfi           0x4    0x0
.debug_line         0x161    0x0
.debug_info          0x22    0x0
.debug_abbrev        0x12    0x0
.debug_aranges       0xb0    0x0
.debug_str           0x30    0x0
.debug_ranges        0xa8    0x0
.debug_frame        0x140    0x0
.ARM.attributes      0x1c    0x0
Total               0x4d9

[rust-highfive]

r? @jonas-schievink

(rust_highfive has picked a reviewer for you, use r? to override)

[japaric]

this subroutine may modify the stack pointer depending on whether MSP or PSP is currently being used so it requires a (rather complex?) register rule so let's not add CFI to it in this PR

[japaric]

like __msp_w this can modify the stack pointer so let's leave it out for now

[japaric]

looks good to me except for the __*sp_w subroutines

[jonas-schievink]

bors r+

[bors]

👎 Rejected by code reviews

[japaric]

Thanks!

[jonas-schievink]

bors r+

[bors]

Build succeeded:

• continuous-integration/travis-ci/push