Mark gglMulx and gglMulAddx as always_inline

Bug: http://b/116873221
Bug: http://b/124451159

This is to get them building with O0 (needed for coverage builds).  At
O0, without the always_inline, the compiler is not able to reconcile the
constraints in the inline assembly.

In file included from system/core/libpixelflinger/trap.cpp:27:
In file included from system/core/libpixelflinger/trap.h:22:
In file included from
system/core/libpixelflinger/include/private/pixelflinger/ggl_context.h:27:
system/core/libpixelflinger/include/private/pixelflinger/ggl_fixed.h:113:9:
error: invalid operand for inline asm constraint 'I'
  asm("smull  %[lo], %[hi], %[x], %[y]            \n"
      ^
system/core/libpixelflinger/include/private/pixelflinger/ggl_fixed.h:136:9:
error: invalid operand for inline asm constraint 'I'
  asm("smull  %[lo], %[hi], %[x], %[y]            \n"
      ^
2 errors generated.

Test: m NATIVE_COVERAGE=true COVERAGE_PATHS=system/core/libpixelflinger
Test: m
Change-Id: Ia4516b19b957687b5fa25d2d100679abcec29366