Fix up final FPU-registers for FreeBSD x86_64.

FreeBSD now builds without compilation errors. For FreeBSD and glory!

                ,        ,
               /(        )`
               \ \___   / |
               /- _  `-/  '
              (/\/ \ \   /\
              / /   | `    \
              O O   ) /    |
              `-^--'`<     '
             (_.)  _  )   /
              `.___/`    /
                `-----' /
   <----.     __ / __   \
   <----|====O)))==) \) /====
   <----'    `--' `.__,' \
                |        |
                 \       /       /\
            ______( (_  / \______/
          ,'  ,-----'   |
          `--{__________)

This closes https://github.com/dotnet/coreclr/issues/594#issuecomment-93995183

src/pal/src/include/pal/context.h

@@ -137,11 +137,7 @@ typedef ucontext_t native_context_t;
 #define FPREG_DataSelector(uc)  *((WORD*) &(FPSTATE(uc)->sv_env.en_rdp) + 2)
 
 #define FPREG_Xmm(uc, index)    *(M128A*) &(FPSTATE(uc)->sv_xmm[index])
-
-// TODO: port this register to FreeBSD.
-// #define FPREG_St(uc, index)     *(M128A*)&((uc)->mc_fpstate[index])
-// something like this?
-// #define FPREG_St(uc, index)     *(M128A*)&(FPSTATE(uc)->sv_xstate.sx_ymm[index])
+#define FPREG_St(uc, index)     *(M128A*) &(FPSTATE(uc)->sv_fp[index].fp_acc)
 
 #else // BIT64