remove old clang workarounds from arm syscall implementation

the register constraints in the non-clang case were tested to work on
clang back to 3.2, and earlier versions of clang have known bugs that
preclude building musl.

there may be other reasons to prefer not to use inline syscalls, but
if so the function-call-based implementations should be added back in
a unified way for all archs.
inline 5- and 6-argument syscalls on arm
unify non-inline version of syscall code across archs

except powerpc, which still lacks inline syscalls simply because
nobody has written the code, these are all fallbacks used to work
around a clang bug that probably does not exist in versions of clang
that can compile musl. however, it's useful to have the generic
non-inline code anyway, as it eases the task of porting to new archs:
writing inline syscall code is now optional. this approach could also
help support compilers which don't understand inline asm or lack
support for the needed register constraints.

mips could not be unified because it has special fixup code for broken
layout of the kernel's struct stat.
arm assembly changes for clang compatibility
fix build regression in arm asm for memcpy

commit 27828f7e9adb6b4f93ca56f6f98ef4c44bb5ed4e fixed compatibility
with clang's internal assembler, but broke compatibility with gas and
the traditional arm asm syntax by switching to the arm "unified
assembler language" (UAL). recent versions of gas also support UAL,
but require the .syntax directive to be used to switch to it. clang on
the other hand defaults to UAL. and old versions of gas (still
relevant) don't support UAL at all.

for the conditional ldm/stm instructions, "ia" is default and can just
be omitted, resulting in a mnemonic that's compatible with both
traditional and UAL syntax. but for byte/halfword loads and stores,
there seems to be no mnemonic compatible with both, and thus .word is
used to produce the desired opcode explicitly. the .inst directive is
not used because it is not compatible with older assemblers.
fix build regression in arm asm for setjmp/longjmp with old assemblers
fix tabs/spaces in memcpy.s

this file had been a mess that went unnoticed ever since it was
imported. some lines used spaces for indention while others used tabs,
and tabs were used for alignment.
adapt dynamic linker for new binutils versions that omit DT_RPATH

the new DT_RUNPATH semantics for search order are always used, and
since binutils had always set both DT_RPATH and DT_RUNPATH when the
latter was used, processing only DT_RPATH worked fine. however, recent
binutils has stopped generating DT_RPATH when DT_RUNPATH is used,
which broke support for this feature completely.