LibC: Make <limits.h> compatible with GCC so that it doesn't install a fixed header

GCC installs a fixed version of the <limits.h> header as per https://gcc.gnu.org/onlinedocs/gcc/Fixed-Headers.html.
The fixed header doesn't include the target's <limits.h> which in turn means that some definitions (such as PATH_MAX)
aren't available. This change requires rebuilding the toolchain (Toolchain/BuildIt.sh).

This fixes the flatbuffers port.

The commit also removes some non-standard defines (U*_MIN) which don't appear to be used
anywhere. By definition they're always 0 though so they're not strictly necessary.

Userland/Libraries/LibC/bits/stdint.h

@@ -74,7 +74,6 @@ typedef __INTPTR_TYPE__ intptr_t;
 
 typedef __UINTMAX_TYPE__ uintmax_t;
 #define UINTMAX_MAX __UINTMAX_MAX__
-#define UINTMAX_MIN __UINTMAX_MIN__
 
 typedef __INTMAX_TYPE__ intmax_t;
 #define INTMAX_MAX __INTMAX_MAX__

Userland/Libraries/LibC/limits.h

@@ -47,13 +47,17 @@
 #define INT_MIN INT32_MIN
 
 #define UINT_MAX UINT32_MAX
-#define UINT_MIN UINT32_MIN
 
 #define CHAR_BIT 8
 #define SCHAR_MIN (-128)
 #define SCHAR_MAX 127
 #define UCHAR_MAX 255
 
+#define SHRT_MAX 32768
+#define SHRT_MIN (-SHRT_MAX - 1)
+
+#define USHRT_MAX 65535
+
 #define LONG_MAX 2147483647L
 #define LONG_MIN (-LONG_MAX - 1L)
 
@@ -62,11 +66,30 @@
 #define LONG_LONG_MAX 9223372036854775807LL
 #define LONG_LONG_MIN (-LONG_LONG_MAX - 1LL)
 
+#define LLONG_MAX LONG_LONG_MAX
+#define LLONG_MIN LONG_LONG_MIN
+
 #define ULONG_LONG_MAX 18446744073709551615ULL
 
 #define CHAR_MIN SCHAR_MIN
 #define CHAR_MAX SCHAR_MAX
 
+#define CHAR_WIDTH 8
+#define SCHAR_WIDTH 8
+#define UCHAR_WIDTH 8
+
+#define SHRT_WIDTH 16
+#define USHRT_WIDTH 16
+
+#define INT_WIDTH 32
+#define UINT_WIDTH 32
+
+#define LONG_WIDTH 32
+#define ULONG_WIDTH 32
+
+#define LLONG_WIDTH 64
+#define ULLONG_WIDTH 64
+
 #define MB_LEN_MAX 16
 
 #define ARG_MAX 65536