Not expecting a fix, just documenting this doesn't seem to work.

Using the precompiled gcore_armv7 binary and codesigning it:

$ codesign -s "iPhone Developer" --entitlements gdb.xcent --force gcore_armv7
gcore_armv7: replacing existing signature

I get the following errors attempting to run on an iPhone 4 on iOS 5.1.1:

uname -a

Darwin iPhone4 11.0.0 Darwin Kernel Version 11.0.0: Wed May 23 10:05:36 PDT 2012; root:xnu-1878.11.10~2/RELEASE_ARM_S5L8930X iPhone3,1 arm N90AP Darwin

./gcore_armv7

usage: gcore [-c ]

./gcore_armv7 -c dump 3225

failed to read target's memory
failed to dump core for process 3225 (4)

Xcode 4.5.1 on Mountain Lion.

$ make gcore_arm
/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/usr/llvm-gcc-4.2/bin//llvm-gcc-4.2 -Os -Wimplicit -isysroot /Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS6.0.sdk -arch armv7 -o gcore_arm gcore.c
In file included from gcore.c:43:
/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS6.0.sdk/usr/include/mach/mach_vm.h:1:2: error: #error mach_vm.h unsupported.
gcore.c: In function ‘X_coredump’:
gcore.c:527: warning: implicit declaration of function ‘mach_vm_region_recurse’
gcore.c:582: warning: implicit declaration of function ‘mach_vm_protect’
gcore.c:601: warning: implicit declaration of function ‘mach_vm_read’
gcore.c:623: warning: implicit declaration of function ‘mach_vm_deallocate’
make: *** [gcore_arm] Error 1