[boot] Provide memset implementation

Clang needs memset, memcpy, etc to exist even in freestanding situations
because it will emit calls to those functions. This commit adds a simple
weak-linked memset implementation.

src/boot/support.cpp

@@ -17,6 +17,18 @@ void *memcpy(void *dest, const void *src, size_t n)
 	return dest;
 }
 
+/// Basic memset() implementation for clang. Clang requires freestanding code
+/// implement memset(), as it may emit references to it. This basic memset is
+/// not the most efficient, but will get linked if no other memcpy exists.
+__attribute__ ((__weak__))
+void *memset(void *dest, int c, size_t n)
+{
+	uint8_t *cdest = reinterpret_cast<uint8_t*>(dest);
+	for (size_t i = 0; i < n; ++i)
+		cdest[i] = static_cast<uint8_t>(c);
+	return dest;
+}
+
 int _purecall()
 {
 	::boot::error::raise(uefi::status::unsupported, L"Pure virtual call");