unix: print a message when a fatal signal happens

Print a message for SIGBUS, SIGSEGV, and SIGILL when they happen.
These signals are always fatal, but it's very useful to know which of
them happened.

Also, it prints the location in the binary which can then be parsed by
`tinygo run` (see https://github.com/tinygo-org/tinygo/pull/4383).

While this does add some extra binary size, it's for Linux and MacOS
(systems that typically have plenty of RAM/storage) and could be very
useful when debugging some low-level crash such as a runtime bug.

src/runtime/arch_mips.go

@@ -9,7 +9,12 @@ const deferExtraRegs = 0
 
 const callInstSize = 8 // "jal someFunc" is 4 bytes, plus a MIPS delay slot
 
-const linux_MAP_ANONYMOUS = 0x800
+const (
+	linux_MAP_ANONYMOUS = 0x800
+	linux_SIGBUS        = 10
+	linux_SIGILL        = 4
+	linux_SIGSEGV       = 11
+)
 
 // It appears that MIPS has a maximum alignment of 8 bytes.
 func align(ptr uintptr) uintptr {