Fix Apple platform detection and iOS cross-build

[moluopro]

This PR fixes Apple platform detection and iOS cross-build instructions.

The main issue is that Apple clang defines TARGET_OS_IPHONE on macOS as well,
but with value 0. The previous detection only checked whether the macro was
defined, so macOS builds could be identified as iOS. This PR checks the macro
value, adds a separate display name for Darwin targets, and updates the iOS CI
and documentation build commands so that iPhoneOS compiler flags are actually
passed to make.

Changes

ctest.c

• Change iOS detection from defined(TARGET_OS_IPHONE) to
  defined(TARGET_OS_IPHONE) && TARGET_OS_IPHONE.
• This avoids treating normal macOS builds as iOS, because Apple headers define
  TARGET_OS_IPHONE as 0 for macOS targets.

c_check

• Replace the boolean is_ios state with darwin_os, which records the Darwin
  target kind as macOS or iOS.
• Keep the internal build OS as Darwin, while writing OSNAME_DISPLAY for
  user-facing output.
• Set CROSS=1 when the detected Darwin target kind is iOS.
• Use grep -q when checking the generated assembly symbol prefix, avoiding
  accidental diagnostic output from the detection command.

c_check.pl

• Mirror the shell c_check behavior in the Perl fallback script.
• Track darwin_os separately from OSNAME.
• Emit OSNAME_DISPLAY only for Darwin targets.
• Set cross-compilation mode for iOS targets.

Makefile

• Add OSNAME_DISPLAY ?= $(OSNAME).
• Print OSNAME_DISPLAY in the final build summary instead of the internal
  OSNAME.
• This keeps existing build logic based on Darwin, while showing users
  macOS or iOS in the summary.

.github/workflows/apple_m.yml

• Update the iOS cross-build step to query the iPhoneOS clang and SDK path with
  xcrun.
• Pass CC and CFLAGS directly to make, instead of setting shell variables
  that make would not necessarily receive.
• Print the resolved CC and SDKROOT values in CI logs for easier debugging.
• Restore DYNAMIC_ARCH=1 in the iOS CI build command.

docs/install.md

• Update the iOS build instructions to match the CI command.
• Use xcrun --sdk iphoneos --find clang instead of a hard-coded Xcode path.
• Pass CC and CFLAGS directly to make.
• Fix the follow-up note to refer to the actual
  -miphoneos-version-min flag instead of MIN_IOS_VERSION.

[moluopro]

I previously fixed the iOS shell script, but didn’t update Perl, CI, or the documentation. I’ve polished those up this time.

[martin-frbg]

Thank you very much