Fix two bugs for musl-libc on ARM

Bug 1: triples like armv7-pc-linux-musl use the wrong linker name
ld-musl-armv7.so.1; the right name should be ld-musl-arm.so.1, disregarding the
subarch field.

Bug 2: when compiler option -mhard-float is used, we should use the "hardfloat"
linker, no matter whether the triple itself mentions "hardfloat".

Patch by Lei Zhang!

Differential Revision: https://reviews.llvm.org/D22904

llvm-svn: 277985