[Hexagon] Generate PIC-specific versions of save/restore routines

In PIC mode, the registers R14, R15 and R28 are reserved for use by
the PLT handling code. This causes all functions to clobber these
registers. While this is not new for regular function calls, it does
also apply to save/restore functions, which do not follow the standard
ABI conventions with respect to the volatile/non-volatile registers.

Patch by Jyotsna Verma.

llvm-svn: 264324