This testcase does not work as expected in QEMU (aarch64 QEMU in my 
case). It fails when trying to manually write the breakpoint instruction 
to a certain PC address.

(gdb) p /x addr_bp[0] = buffer[0]^M
Cannot access memory at address 0x400834^M
(gdb) PASS: gdb.base/bp-permanent.exp: always_inserted=off, 
sw_watchpoint=0: setup: p /x addr_bp[0] = buffer[0]
p /x addr_bp[1] = buffer[1]^M
Cannot access memory at address 0x400835^M
(gdb) PASS: gdb.base/bp-permanent.exp: always_inserted=off, 
sw_watchpoint=0: setup: p /x addr_bp[1] = buffer[1]
p /x addr_bp[2] = buffer[2]^M
Cannot access memory at address 0x400836^M
(gdb) PASS: gdb.base/bp-permanent.exp: always_inserted=off, 
sw_watchpoint=0: setup: p /x addr_bp[2] = buffer[2]
p /x addr_bp[3] = buffer[3]^M
Cannot access memory at address 0x400837^M
(gdb) PASS: gdb.base/bp-permanent.exp: always_inserted=off, 
sw_watchpoint=0: setup: p /x addr_bp[3] = buffer[3]

The following patch prevents a number of failures by detecting this and 
bailing out in case the target has such a restriction. Writing to .text 
from within the program isn't any better. It just leads to a SIGSEGV.

Before the patch:

                 === gdb Summary ===

# of expected passes            66
# of unexpected failures        56

After the patch:

                 === gdb Summary ===

# of expected passes            26
# of unsupported tests          4

I regtested this on x86-64 and it still works fine.

Ok?