GDB is able to cache memory accesses requested in target_read_code,
so target_read_code is more efficient than general target_read_memory.
This patch uses target_read_code and its variants to read target
memory in the functions related to i386_skip_prologue. It improves
the performance when doing 'b foo' (foo is a function) in remote
debugging.
Nowadays, when we set a breakpoint on function f1, GDB will fetch the
code in f1 to determine the start of the function body (say skip the
prologue), it requests read from target many times. With this patch
applied, the number of RSP 'm' packets are reduced.