Check for MIPSEB in target.h (aws#2143)

### Issues:
Addresses: 
* aws/aws-lc-rs#522

### Description of changes: 
Setup macros for big-endian MIPS in "target.h"

### Callout
A contributor informs us that the build for `mips-unknown-linux-musl`
succeeds with this change:
aws/aws-lc-rs#522 (comment)

### Testing:
No CI testing for it. 

By submitting this pull request, I confirm that my contribution is made
under the terms of the Apache 2.0 license and the ISC license.

include/openssl/target.h

@@ -55,6 +55,10 @@
 #elif defined(__MIPSEL__) && defined(__LP64__)
 #define OPENSSL_64_BIT
 #define OPENSSL_MIPS64
+#elif defined(__MIPSEB__) && !defined(__LP64__)
+#define OPENSSL_32_BIT
+#define OPENSSL_MIPS
+#define OPENSSL_BIG_ENDIAN
 #elif defined(__riscv) && __SIZEOF_POINTER__ == 8
 #define OPENSSL_64_BIT
 #define OPENSSL_RISCV64