upgrade to musl 1.2.0

[andrewrk]

musl 1.2.0 was released. Time to follow the update procedure.

Previous issue: #3463

Take special care to read the detailed time64 release notes. Some of the structs for 32-bit architectures may need to be adjusted to match a new ABI.

Related issues:

• add support for targeting glibc 2.31 #4459 upgrade to glibc 2.31
• upgrade to mingw-w64 7.0.0 #3713 upgrade to mingw-w64 7.0.0

Currently this is done in stage1 C++ code. However I will be looking into self-hosting building libc (#4313) before solving this issue.

1.2.0 release notes

new features:

• time_t is now 64-bit on all archs (not just 64-bit archs)
• character type & case mapping data updated to Unicode 12.1.0
• header-level support for new linux features in 5.3 and 5.4

performance:

• new O(1) wchar_t case mapping implementation
• i386 now uses C math code for exp, faster than old asm
• mips math asm

compatibility & conformance:

• endian.h now aims to conform to future POSIX definition
• support older compilers that don't accept powerpc math asm constraints
• fdpic code in ldso was incompatible with valid optimizations in gcc 9+
• RLIMIT_RTTIME was missing from sys/resource.h

bugs fixed:

• wcwidth wrongly returned 0 for most of planes 4 and up
• missing case mapping between U+03F3 and U+037F
• wrong cacosh results for arguments with negative imaginary part
• wrong catanf/catanl results for various classes of arguments
• wrong return value for ungetc with argument outside [0,UCHAR_MAX]
• posix_openpt with no ptys available produced wrong errno

arch-specific bugs fixed:

• sigcontext/regset definition mistakes & omissions on m68k, powerpc64
• fesetenv(FE_DFL_ENV) crashed on riscv64
• sh2 dynamic linker was broken since 1.1.21 (crash in stage 2b)
• arm dynamic linker chose wrong tls/atomic variants since 1.1.21
• some math library functions returned excess precision on i386
• unconfirmed regression in fchmodat AT_SYMLINK_NOFOLLOW on mips*