Fix MonoContextSimdReg usage on non-glibc Linux amd64(mono#6007)

Include emmintrin.h and use __m128d for MonoContextSimdReg on Linux.

Fixes breakage on Alpine Linux introduced in mono#5764

Author: joncham

mono/utils/mono-context.h

@@ -36,6 +36,9 @@ typedef struct _libc_xmmreg MonoContextSimdReg;
 typedef __m128d MonoContextSimdReg;
 #elif defined(HOST_ANDROID)
 typedef struct _libc_xmmreg MonoContextSimdReg;
+#elif defined(__linux__)
+#include <emmintrin.h>
+typedef __m128d MonoContextSimdReg;
 #endif
 #elif defined(TARGET_ARM64)
 typedef __uint128_t MonoContextSimdReg;