Introduce platform-agnostic `ALWAYS_INLINE` macro

`<attributes.h>` has been included in anticipation of the following
commit.

src/attributes.h

@@ -16,4 +16,12 @@
 #  define LIFETIMEBOUND
 #endif
 
+#if defined(__GNUC__)
+#  define ALWAYS_INLINE inline __attribute__((always_inline))
+#elif defined(_MSC_VER)
+#  define ALWAYS_INLINE __forceinline
+#else
+#  error No known always_inline attribute for this platform.
+#endif
+
 #endif // BITCOIN_ATTRIBUTES_H

src/crypto/sha256_avx2.cpp

@@ -7,6 +7,7 @@
 #include <stdint.h>
 #include <immintrin.h>
 
+#include <attributes.h>
 #include <crypto/common.h>
 
 namespace sha256d64_avx2 {

src/crypto/sha256_sse41.cpp

@@ -7,6 +7,7 @@
 #include <stdint.h>
 #include <immintrin.h>
 
+#include <attributes.h>
 #include <crypto/common.h>
 
 namespace sha256d64_sse41 {

src/crypto/sha256_x86_shani.cpp

@@ -11,6 +11,8 @@
 #include <stdint.h>
 #include <immintrin.h>
 
+#include <attributes.h>
+
 namespace {
 
 alignas(__m128i) const uint8_t MASK[16] = {0x03, 0x02, 0x01, 0x00, 0x07, 0x06, 0x05, 0x04, 0x0b, 0x0a, 0x09, 0x08, 0x0f, 0x0e, 0x0d, 0x0c};