This fixes an error in 6af4f25b89: The
r0 register is special in addressing modes on s390x and is interpreted
as constant zero, i.e. lg %r5, 8(%r0) would effectively become lg %r5,
8. So care should be taken to never use r0 as an address register in
s390x assembly.
per the psABI, floating point register contents beyond the register
size of the targeted ABI variant are never call-saved, so no
hwcap-conditional logic is needed here and the assembly-time
conditions are based purely on ABI variant macros, not the targeted
ISA level.
This just mirrors what is done in the start code for the affected
ports, as well as what is already done for the three x86 ports.
Clearing the frame pointer helps protect FP-based unwinders from
wrongly attempting to traverse into the parent thread's call frame
stack.
Calling __tls_get_addr with brasl is not valid since it's a global symbol; doing
so results in an R_390_PC32DBL relocation error from lld. We could fix this by
marking __tls_get_addr hidden since it is not part of the s390x ABI, or by using
a different instruction. However, given its simplicity, it makes more sense to
just manually inline it into __tls_get_offset for performance.
The patch has been tested by applying to Zig's bundled musl copy and running the
full Zig test suite under qemu-s390x.
The LLVM assembler reportedly assembles the form using the j mnemonic
incorrectly (see issue 107460). The jr form is canonical and avoids
this problem, so use it instead.
