s390x regtest: Adjust various test cases for Clang

s390x regtest: Adjust various test cases for Clang

There are various issues when compiling the s390x test cases with
Clang (see Bug 465782):

* Bugs in inline assemblies tolerated/obscured by GCC:

  - "d" (or "r") constraint instead of "a" where a register from 1-15 is
    needed
  - mix-up between vector register and general register in inline assembly
  - too unspecific constraints such as "m" or "g"
  - use of input operand for output
  - missing register clobber declaration

* Missing Clang features:

  - "%R" (register) and "%O" (offset) modifiers with address constraints
    in inline assemblies
  - variable-length array within a struct

* Use of the -mlong-double-128 command line option -- has been redundant
  for some time with GCC, and Clang only supports 128-bit long double

* Bad definition of _FPU_SETCW/_FPU_GETCW in <fpu_control.h>
  (See https://sourceware.org/bugzilla/show_bug.cgi?id=30130)

* Use of pow() function without linking libm -- GCC optimizes the call
  away, while Clang keeps a reference to pow()

Perform appropriate changes to the test cases such that they can be
compiled both with Clang and with GCC.